Magnetic disk device and head position correction method

ABSTRACT

According to one embodiment, a magnetic disk device includes a disk including a recording region including a servo sector, a head configured to write data to the disk and read data from the disk, and a controller configured to acquire a plurality of correction data for a repeatable runout of the recording region, the correction data respectively corresponding to a plurality of measurement positions set based on a first linearity error acquired by reading the servo sector, and to correct a position of the head based on the correction data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2018-174650, filed Sep. 19, 2018, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a magnetic disk deviceand a head position correction method.

BACKGROUND

In magnetic disk devices, a technology of suppressing an error caused byrepeatable runout (RRO) (hereinafter the error is simply referred to asRRO) to correct a position of a head has been developed. For example,there is a method of measuring the RRO at a plurality of differentpositions in a radial direction of a disk and correcting the position ofthe head based on data obtained by interpolating the RRO among aplurality of measured data. In the method of correcting the position ofthe head, it is necessary to appropriately set the position to measurethe RRO.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a magneticdisk device according to a first embodiment;

FIG. 2 is a schematic diagram illustrating an example of an arrangementof a head with respect to a disk according to the embodiment;

FIG. 3 is a schematic diagram illustrating an example of a configurationof a servo region;

FIG. 4 is a diagram illustrating an example of a Lissajous waveform of ademodulated signal obtained by demodulating N burst and a demodulatedsignal obtained by demodulating Q burst;

FIG. 5 is a diagram illustrating an example of a relationship between atarget index and a measurement index of a servo demodulation criterion;

FIG. 6 is a schematic diagram illustrating an example of a linearityerror;

FIG. 7 is a diagram illustrating an example of distribution ofpositioning errors corresponding to linear learning positions and thelinearity errors;

FIG. 8 is a diagram illustrating an example of a method of setting alinear learning position according to the present embodiment;

FIG. 9 is a flowchart illustrating an example of a method of determininga learning position in linear RRO correction processing according to thepresent embodiment;

FIG. 10 is a flowchart illustrating an example of the method of settinga linear learning position according to the present embodiment;

FIG. 11 is a diagram illustrating an example of variation in thelinearity error;

FIG. 12 is a flowchart illustrating an example of a method of setting alinear learning position according to a first modification;

FIG. 13 is a diagram illustrating an example of a variation periodiccomponent of a linearity error;

FIG. 14 is a schematic diagram illustrating an example of a method ofsetting a linear learning position according to a second modification;and

FIG. 15 is a flowchart illustrating an example of a method of setting alinear learning position based on linearity correction according to thesecond modification.

DETAILED DESCRIPTION

In general, according to one embodiment, a magnetic disk devicecomprises: a disk including a recording region including a servo sector;a head configured to write data to the disk and read data from the disk;and a controller configured to acquire a plurality of correction datafor a repeatable runout of the recording region, the correction datarespectively corresponding to a plurality of measurement positions setbased on a first linearity error acquired by reading the servo sector,and to correct a position of the head based on the correction data.

According to another embodiment, a magnetic disk device comprises: adisk including a recording region including a servo sector; a headconfigured to write data to the disk and read data from the disk; and acontroller configured to acquire a plurality of correction data for arepeatable runout of the recording region, the correction datarespectively corresponding to a plurality of measurement positions setbased on a Lissajous waveform acquired by reading the servo sector, andto correct a position of the head based on the correction data.

According to another embodiment, a head position correction methodapplied to a magnetic disk device including a disk including a recordingregion including a servo sector and a head configured to write data tothe disk and read data from the disk, the method comprises: acquiring aplurality of correction data for a repeatable runout of the recordingregion, the correction data respectively corresponding to a plurality ofmeasurement positions set based on a first linearity error acquired byreading the servo sector, and correcting a position of the head based onthe correction data.

Hereinafter, embodiments will be described with reference to thedrawings. Note that the drawings are merely examples and do not limitthe scope of the invention.

Embodiment

FIG. 1 is a block diagram illustrating a configuration of a magneticdisk device 1 according to an embodiment.

The magnetic disk device 1 includes a head disk assembly (HDA), a driverIC 20, a head amplifier integrated circuit (hereinafter, head amplifierIC or preamplifier) 30, a volatile memory 70, a nonvolatile memory 80, abuffer memory (buffer) 90, and a system controller 130 that is aone-chip integrated circuit, described below. Further, the magnetic diskdevice 1 is connected to a host system (hereinafter simply referred toas a host) 100.

The HDA includes a magnetic disk (hereinafter referred to as a disk) 10,a spindle motor (hereinafter referred to as an SPM) 12, an arm 13 onwhich a head 15 is mounted, a voice coil motor (hereinafter referred toas VCM) 14. The disk 10 is attached to the SPM 12 and rotates by drivingof the SPM 12. The arm 13 and the VCM 14 constitute an actuator. Theactuator controls the movement of the head 15 mounted on the arm 13 to aparticular position on the disk 10 by driving of the VCM 14. Two or morenumbers of the disk 10 and the head 15 may be provided.

A user data region 10 a usable by a user and a system area 10 b forwriting information necessary for system management are allocated to adata writable region in the disk 10. Hereinafter, a direction orthogonalto a radial direction of the disk 10 is referred to as a circumferentialdirection.

The head 15 includes a write head 15W and a read head 15R mounted on aslider as a main body. The write head 15W writes data on the disk 10.The read head 15R reads data recorded in a track on the disk 10. Notethat the write head 15W may be simply referred to as a head 15, the readhead 15R may be simply referred to as a head 15, and the write head 15Wand the read head 15R may be collectively referred to as a head 15 insome cases. Hereinafter, a central portion of the head 15 may bereferred to as a head 15, a central portion of the write head 15W may bereferred to as a write head 15W, and a central portion of the read head15R may be referred to as a read head 15R in some cases. The “track” isused as one of a plurality of regions divided in the radial direction ofthe disk 10, data extending in the circumferential direction of the disk10, data written in the track, and various other meanings. A “sector” isused as one of a plurality of regions obtained by dividing the track inthe circumferential direction, data written at a particular position onthe disk 10, data written in the sector, and various other meanings.Further, a width in the radial direction of the track is referred to asa track width, and a center position of a target track width is referredto as a track center. The track center is, for example, a perfect circlearranged concentrically with the disk 10.

FIG. 2 is a schematic diagram illustrating an example of an arrangementof a head 15 with respect to a disk 10 according to the embodiment. InFIG. 2, a direction toward an outer periphery of the disk 10 in theradial direction is referred to as an outward direction (outer side),and a direction opposite to the outward direction is referred to as aninward direction (inner side). Further, FIG. 2 illustrates a rotationdirection of the disk 10. Note that the rotation direction may be areverse direction. In FIG. 2, the user data region 10 a is divided intoan inner peripheral region IR located in the inward direction, an outerperipheral region OR located in the outward direction, and a middleperipheral region MR located between the inner peripheral region IR andthe outer peripheral region OR. FIG. 2 illustrates a path of a trackcenter (hereinafter simply referred to as a track center) IIL of aparticular track of the inner peripheral region IR, a track center IL0of a particular track of the middle peripheral region MR, and a trackcenter OIL of a particular track of the outer peripheral region OR. Thetrack center IIL corresponds to a path to be targeted of the head 15 ina case where the head 15 is positioned at a particular track of theinner peripheral region IR (hereinafter, the path to be targeted isreferred to as a target path or a target trajectory). The track centerIL0 corresponds to a target path of the head 15 in a case where the head15 is positioned at a particular track of the middle peripheral regionMR. The track center OIL corresponds to a target path of the head 15 ina case where the head 15 is positioned at a particular track of theouter peripheral region OR. Further, FIG. 2 illustrates paths ISL, SL0,and OSL of the head 15 deviated from the track centers IIL, IL0, and OILdue to repeatable runout (RRO).

The disk 10 includes a plurality of servo regions SV. Hereinafter, theservo region SV may be referred to as a servo sector in some cases. Theplurality of servo regions SV radially extends in the radial directionof the disk 10 and is discretely arranged with a particular interval inthe circumferential direction. The servo region SV includes servo dataand RRO correction data for positioning the head 15 at a particularposition in the radial direction (hereinafter, the position is referredto as a radial position) of the disk 10.

The servo data includes, for example, a servo mark, address data, burstdata, and the like. The address data includes an address (cylinderaddress) of a particular track and an address of a servo sector of aparticular track. The burst data is data (relative position data) usedfor detecting a positional deviation (position error) in the radialdirection of the head 15 with respect to the track center of aparticular track, and includes a repetitive pattern of a particularperiod. The burst data is written in a zigzag manner across externallyadjacent tracks. The burst data includes an error caused by a distortionof a track with respect to a track (track center) of a perfect circlecaused by blurring (repeatable runout (RRO)) synchronized with therotation of the disk 10 when the servo data is written to the disk.Hereinafter, for the sake of convenience of description, the errorcaused by the distortion of the track with respect to the track centercaused by the RRO is simply referred to as RRO.

In each of the plurality of servo regions SV, a pattern constituting RROcorrection data for correcting the RRO (hereinafter, the pattern issimply referred to as RRO correction data) is written. The RROcorrection data is a kind of additional data of the servo data. The RROcorrection data is used for correcting the RRO of the servo data (morespecifically, the servo burst data in the servo data), that is,correcting the distortion of the path of the head 15 with respect to thetrack center. The correction of the RRO may be referred to as perfectcircle correction in some cases.

The RRO correction data includes a RRO preamble pattern, asynchronization pattern, and digital data (hereinafter referred to asRRO correction code (RRO code)) obtained by encoding a correctionamount. The RRO preamble pattern and the synchronization pattern areused for detecting read start timing of the digital data obtained byencoding the correction amount to be written in a subsequent region. Atthis time, the RRO correction code (RRO code) constitutes a main part ofthe RRO correction data. Such RRO correction data may be referred to asRRO bit or PostCode (post code) in some cases.

In the example illustrated in FIG. 2, in a case where the head 15 ispositioned at the track center IIL of a particular track in the innerperipheral region IR, the head 15 is corrected to move from the path ISLto over the track center IIL based on the servo data of the servo regionSV of the disk 10. In a case where the head 15 is positioned at thetrack center IL0 of a particular track in the middle peripheral regionMR, the head 15 is corrected to move from the path SL0 to over the trackcenter IL0 based on the servo data of the servo region SV of the disk10. In a case where the head 15 is positioned at the track center OIL ofa particular track in the outer peripheral region OR, the head 15 iscorrected to move from the path OSL to over the track center OIL basedon the servo data of the servo region SV of the disk 10.

The driver IC 20 controls driving of the SPM 12 and the VCM 14 accordingto control of the system controller 130 (more specifically, an MPU 60described below).

The head amplifier IC (preamplifier) 30 includes a read amplifier and awrite driver. The read amplifier amplifies a read signal read from thedisk 10 and outputs the amplified read signal to the system controller130 (more specifically, a read/write (R/W) channel 40 described below).The write driver outputs a write current corresponding to a signaloutput from the R/W channel 40 to the head 15.

The volatile memory 70 is a semiconductor memory in which stored data islost when a power supply is cut off. The volatile memory 70 stores dataand the like necessary for processing in each part of the magnetic diskdevice 1. The volatile memory 70 is, for example, a dynamic randomaccess memory (DRAM) or a synchronous dynamic random access memory(SDRAM).

The nonvolatile memory 80 is a semiconductor memory in which stored datais recorded even when the power supply is cut off. The nonvolatilememory 80 is, for example, a NOR-type or NAND-type flash read onlymemory (flash ROM: FROM).

The buffer memory 90 is a semiconductor memory that temporarily recordsdata and the like transmitted and received between the magnetic diskdevice 1 and the host 100. Note that the buffer memory 90 may beintegrally configured with the volatile memory 70. The buffer memory 90is, for example, a DRAM, a static random access memory (SRAM), an SDRAM,a ferroelectric random access memory (FeRAM), a magnetoresistive randomaccess memory (MRAM), or the like.

The system controller (controller) 130 is realized by using a largescale integrated circuit (LSI) called system-on-a-chip (SoC) in which aplurality of elements is integrated on a single chip, for example. Thesystem controller 130 includes the read/write (R/W) channel 40, a harddisk controller (HDC) 50, and a microprocessor (MPU) 60. The systemcontroller 130 is electrically connected to, for example, the driver IC20, the head amplifier IC 30, the volatile memory 70, the nonvolatilememory 80, the buffer memory 90, and the host 100.

The R/W channel 40 executes signal processing of read data transferredfrom the disk 10 to the host 100 and write data transferred from thehost 100 in response to an instruction from the MPU 60 described below.The R/W channel 40 has a circuit or a function to measure signal qualityof the read data. The R/W channel 40 is electrically connected to, forexample, the head amplifier IC 30, the HDC 50, the MPU 60, and the like.

The HDC 50 controls data transfer between the host 100 and the R/Wchannel 40 in response to an instruction from the MPU 60 to be describedbelow. The HDC 50 is electrically connected to, for example, the R/Wchannel 40, the MPU 60, the volatile memory 70, the nonvolatile memory80, the buffer memory 90, and the like.

The MPU 60 is a main controller that controls each part of the magneticdisk device 1. The MPU 60 controls the VCM 14 via the driver IC 20 andexecutes servo control for positioning the head 15. In addition, the MPU60 controls the SPM 12 via the driver IC 20 to rotate the disk 10. TheMPU 60 controls a write operation of data to the disk 10 and selects asave destination of the write data. Further, the MPU 60 controls a readoperation of data from the disk 10 and controls processing of the readdata. The MPU 60 is connected to each part of the magnetic disk device1. The MPU 60 is electrically connected to, for example, the driver IC20, the R/W channel 40, the HDC 50, and the like.

The MPU 60 includes a read/write controller 610, an RRO learning unit620, an RRO recorder 630, and a position corrector 640. The MPU 60executes processing of these units, for example, the read/writecontroller 610, the RRO learning unit 620, the RRO recorder 630, theposition corrector 640, and the like on firmware. Note that the MPU 60may include these units, for example, the read/write controller 610, theRRO learning unit 620, the RRO recorder 630, and the position corrector640, as circuits.

The read/write controller 610 controls data read processing and datawrite processing according to a command from the host 100. Theread/write controller 610 controls the VCM 14 via the driver IC 20,positions the head 15 at a particular position on the disk 10, and readsor writes data. Hereinafter, “positioning or arranging the head 15 (thewrite head 15W and the read head 15 R) at a particular position on thedisk 10, for example, at a position to be targeted (hereinafter referredto as a target position) of a particular track” may be described as“positioning or arranging the head 15 (the write head 15W and the readhead 15R) at a particular track” in some cases.

The RRO learning unit 620 positions the read head 15R at a particularposition on the disk 10, for example, at the track center of aparticular track, measures a difference value (hereinafter referred toas an RRO correction amount) between the track center and the positionof the head 15 (read head 15R) demodulated from the read servo data, andcalculates RRO correction data from a measurement result. Hereinafter,“measuring the RRO correction amount” and “calculating the RROcorrection data based on the RRO correction amount” are referred to as“RRO learning”. The “RRO learning” may be simply referred to as“measuring”, “reading”, “acquiring”, or the like. In some cases, the RROcorrection amount and the RRO correction data may be used in the samemeaning. A particular radial position at which the RRO learning isexecuted and a particular radial position at which the RRO learning hasbeen executed may be referred to as a learning position in some cases.For example, the learning position corresponds to a distance in theradial direction from the track center of a particular track. Further,the RRO learning unit 620 can acquire RRO learned position informationin the circumferential direction RRO, and the like. For example, the RROlearning unit 620 executes RRO learning processing at a test stage or aproduct stage of the magnetic disk device 1. Note that the RRO learningunit 620 may execute the RRO learning at some positions in thecircumferential direction at a particular radial position or may executethe RRO learning at all positions in the circumferential direction.Further, the RRO learning unit 620 may execute the RRO learning atseveral radial positions or may execute the RRO learning at all radialpositions on the disk 10.

The RRO learning unit 620 estimates change in the radial direction ofthe RRO correction amount of the disk 10 (hereinafter, the change isreferred to as change of the RRO or change of the RRO correction amount)in a particular region in the radial direction of the disk 10(hereinafter, the particular region is referred to as radial region)based on a plurality of the RRO correction amounts respectivelycorresponding to a plurality of the learning positions, and executes theRRO learning at a plurality of the radial positions of the radial regionof the disk 10 in order to correct the radial position of the head 15based on the estimated change of the RRO correction amount of the radialregion. Hereinafter, the radial position of the head 15 may be simplyreferred to as a head position. For example, a slope of the change ofthe RRO may vary from track to track. The RRO learning unit 620 executesthe RRO learning at a plurality of radial positions of the radial regionof the disk 10, where processing of estimating change of the RROcorrection amount of the region based on two RRO correction amountsrespectively acquired at two learning positions, and correcting theradial position of the head 15 based on the estimated change of the RROcorrection amount is executable. Hereinafter, the “processing ofestimating change of the RRO correction amount of the region based ontwo RRO correction amounts respectively acquired at two learningpositions in the radial region, and correcting the head position basedon the estimated change of the RRO correction amount” may be referred toas “linear RRO correction processing” in some cases. Note that, in thelinear RRO correction processing, the change of the RRO correctionamount of the region may be estimated based on three or more RROcorrection amounts respectively acquired at three learning positions inthe radial region, and the head position may be corrected based on theestimated change of the RRO correction amounts.

The RRO learning unit 620 sets the learning position (hereinafterreferred to as linear learning position) used in the linear RROcorrection processing based on a difference value between informationcorresponding to an arrangement in the radial direction of an idealradial region (or a particular track), for example, of two adjacenttracks (the information may be referred to as ideal servo demodulationcriterion), and information corresponding to an arrangement in theradial direction of the particular track acquired by measurement (theinformation may be referred to as actual servo demodulation criterion)in order to improve the accuracy of the linear RRO correctionprocessing. The arrangement in the radial direction of the radial region(or a particular track) includes, for example, a distortion in theradial direction of the radial region (or the particular track) withrespect to the track center (hereinafter, the distortion may be referredto as nonlinearity of the servo demodulation criterion). Hereinafter,the difference value between the information corresponding to thearrangement in the radial direction of the ideal radial region and theinformation corresponding to the arrangement in the radial direction ofthe radial region acquired by measurement is referred to as a linearityerror. For example, the linearity error is an index indicating adistortion of the radial region, for example, the particular track.

The RRO learning unit 620 includes a function to correct the linearityerror in a process of reading a signal of the servo region SV andcalculating a servo demodulation position. Hereinafter, “correcting thelinearity error” is referred to as “linearity correction”. The RROlearning unit 620 adjusts various parameters (hereinafter referred to ascorrection parameters) corresponding to the linearity correction in theprocess of calculating the servo demodulation position and executes thelinearity correction. The RRO learning unit 620 may record thecorrection parameters adjusted when executing the linearity correctionin a particular recording region, for example, the disk 10 and thenonvolatile memory 80. The RRO learning unit 620 executes the linearitycorrection, for example, in a manufacturing process. For example, in acase where the linearity correction is not appropriate in themanufacturing process, the linearity error may become large. The RROlearning unit 620 sets, for example, the linear learning position basedon the correction parameters.

Hereinafter, the linearity error will be described with reference toFIGS. 3 to 6.

FIG. 3 is a schematic diagram illustrating an example of a configurationof the servo region SV. FIG. 3 illustrates a track TRn and a track TRn+1that are arranged in succession in the radial direction. The track TRnhas a track center TRCn. The track TRn+1 has a track center TRCn+1. Notethat, for convenience of description, the tracks TRn and TRn+1 linearlyextend in the circumferential direction, but in practice the tracks TRnand TRn+1 are curved along the circumferential direction of the disk 10.Each of the tracks TRn and TRn+1 may extend in the circumferentialdirection in a wavy manner while periodically varying. In addition, thetracks TRn and TRn+1 may be slightly spaced apart in the radialdirection or may be partially overlapped.

In the example illustrated in FIG. 3, the servo region SV includespreamble, servo mark, gray code, PAD, N burst, Q burst, and post code.The preamble includes preamble information for being synchronized with areproduction signal of a servo pattern. The servo mark includes servomark information indicating the start of the servo pattern. The graycode includes gray code information indicating a servo sector number, atrack (cylinder) number, and the like. The PAD includes PAD informationof synchronization signals of gap and servo AGC. The N burst and the Qburst include burst information indicating a relative position in theradial direction of the head 15 (the write head 15W and the read head15R) with respect to the track. The post code includes the RROcorrection data. Note that the post code need not be included in theservo region SV.

In the example illustrated in FIG. 3, the RRO learning unit 620demodulates the gray code, the N burst, the Q burst, and the PostCodefollowing the servo mark information in the circumferential direction,the servo mark information having been read by the read head 15R at theradial position of the servo mark, for example, at the track center TRCnof the track TRn, and detects the demodulated radial position of theread head 15R as the servo demodulation position. The RRO learning unit620 may record information of the detected servo demodulation positionand the like in a particular recording region, for example, the disk 10and the nonvolatile memory 80.

FIG. 4 is a diagram illustrating an example of a Lissajous waveform of ademodulated signal obtained by demodulating the N burst and ademodulated signal obtained by demodulating the Q burst. In FIG. 4, thehorizontal axis represents the demodulated signal obtained bydemodulating an N burst signal read by the read head 15 R at aparticular radial position (hereinafter, the demodulated signal isreferred to as an N burst demodulated signal), and the vertical axisrepresents a demodulated signal obtained by demodulating a Q burstsignal read by the read head 15R at a particular radial position(hereinafter referred to as a Q burst demodulated signal). The N burstdemodulated signal corresponds to, for example, a demodulation positionat which the N burst is demodulated, and corresponds to an amount ofdeviation in the radial direction from the track center (or the targetposition) of the track corresponding to the demodulated N burst(hereinafter, the amount of deviation is referred to as an off-trackamount). The Q burst signal corresponds to, for example, a demodulationposition at which the Q burst is demodulated, and corresponds to theoff-track amount from the track center (or the target position) of thetrack corresponding to the demodulated Q burst. On the horizontal axisin FIG. 4, the demodulated signal becomes larger in the direction ofpositive values as the axis goes in the direction of the positive arrowfrom the origin 0, and the demodulated signal becomes smaller in thedirection of negative values as the axis goes in the direction of thenegative arrow from the origin 0. On the vertical axis in FIG. 4, thedemodulated signal becomes larger in the direction of positive values asthe axis goes in the direction of the positive arrow from the origin 0,and the demodulated signal becomes smaller in the direction of negativevalues as the axis goes in the direction of the negative arrow from theorigin 0. FIG. 4 illustrates a Lissajous waveform (or Lissajous figure)LF. One round of the Lissajous waveform LF corresponds to informationcorresponding to the radial positions in the radial region correspondingto two servo tracks, and for example, corresponds to informationcorresponding to the radial positions in the radial region of the tracksTRn and TRn+1 illustrated in FIG. 3.

In the example illustrated in FIG. 4, the Lissajous waveform LF has asubstantially circular shape. In a case where the Lissajous waveform LFhas a circular shape, the linearity error can be small. In a case wherethe Lissajous waveform LF has a rectangular shape, the linearity errorcan be large. The RRO learning unit 620 may acquire the Lissajouswaveform LF based on the demodulated signals obtained by demodulatingdata read at the radial positions in the radial region of at least twoservo tracks. Further, the RRO learning unit 620 may record the acquiredLissajous waveform LF in a particular recording region, for example, thedisk 10 and the nonvolatile memory 80.

FIG. 5 is a diagram illustrating an example of a relationship between atarget index and a measurement index of the servo demodulationcriterion. In FIG. 5, the horizontal axis represents a servo off-trackamount to be targeted in the radial region (hereinafter referred to as atarget index), and the vertical axis represents an actual servooff-track amount based on the Q burst demodulated signal and the N burstdemodulated signal obtained by measurement in the radial region(hereinafter, the actual servo off-track amount is referred to asmeasurement index). On the horizontal axis, the target index becomeslarger as the axis goes along the arrow of large, and the target indexbecomes smaller as the axis goes along the arrow of small. On thevertical axis, the target index becomes larger as the axis goes alongthe arrow of large, and the target index becomes smaller as the axisgoes along the arrow of small. FIG. 5 illustrates a line LL51 and abroken line LL52. The line LL51 and the broken line LL52 indicate arelationship between the target index and the measurement index.

In the example illustrated in FIG. 5, the line LL51 indicates that themeasurement index and the target index are in a proportionalrelationship. That is, the line LL51 indicates that no linearity errorhas occurred. The broken line LL52 indicates that the measurement indexand the target index are in a nonlinear relationship. That is, thebroken line LL52 indicates that the linearity error has occurred. TheRRO learning unit 620 calculates, for example, the measurement indexbased on the Q burst demodulated signal and the N burst demodulatedsignal and calculates the relationships LL51 and LL52 between the targetindex and the measurement index based on the calculated measurementindex and the target index.

FIG. 6 is a schematic diagram illustrating an example of the linearityerror. In FIG. 6, the horizontal axis represents the target index andthe vertical axis represents the linearity error. On the horizontalaxis, the target index becomes larger as the axis goes along the arrowof large, and the target index becomes smaller as the axis goes alongthe arrow of small. On the vertical axis, the linearity error becomeslarger in the direction of positive values as the axis goes along thepositive arrow from the origin 0, and the linearity error becomessmaller in the direction of negative values as the axis goes along thenegative arrow from the origin 0. FIG. 6 illustrates a line LL61 and abroken line LL62. The line LL61 and the broken line LL62 indicate arelationship between the target index and the linearity error. The lineLL61 corresponds to the line LL51 illustrated in FIG. 5, and the brokenline LL62 corresponds to the broken line LL52 illustrated in FIG. 5.

In the example illustrated in FIG. 6, in a case where the linearityerror occurs as indicated by the relationship LL62 between the targetindex and the linearity error, the RRO learning unit 620 executes thelinearity correction. The RRO learning unit 620 adjusts variouscorrection parameters to adjust the linearity error. The RRO learningunit 620 records the various correction parameters in a particularrecording region, for example, the disk 10 and the nonvolatile memory80.

FIG. 7 is a diagram illustrating an example of distribution ofpositioning errors corresponding to the linear learning positions andthe linearity errors. In FIG. 7, the horizontal axis represents theradial position of a track TRk with respect to a track center RCG, andthe vertical axis represents a servo positioning error (repeatableposition error (RPE)) of a case of positioning the head 15 at aparticular radial position of the track TRk based on the RRO correctionamount acquired by the linear RRO correction processing at the trackTRk. On the horizontal axis, the radial position is located on the outerperiphery side of the disk 10 as the axis goes along the arrow of theoutward direction, and the radial position is located on the innerperiphery side of the disk 10 as the axis goes along the arrow of theinward direction. On the vertical axis, the positioning error becomeslarger as the axis goes along the arrow of large, and the positioningerror becomes smaller as the axis goes along the arrow of small. FIG. 7illustrates the track center RCG of the track TRk, a linear learningposition px11 located in the outward direction from the track centerRCG, a linear learning position px21 located in the inward directionfrom the track center RCG, a linear learning position px12 located inthe outward direction from the track center RCG and in the inwarddirection from the linear learning position px11, and a linear learningposition px22 located in the inward direction from the track center RCGand in the outward direction from the linear learning position px21.FIG. 7 illustrates distribution D1 of the positioning errors,distribution D2 of the positioning errors, and distribution D3 of thepositioning errors.

In the example illustrated in FIG. 7, the distribution D1 of thepositioning errors illustrates change of the positioning error in a caseof positioning the head 15 at the radial positions of the track TRkbased on the RRO correction amounts corresponding to the radialpositions of the track TRk acquired by the linear RRO correctionprocessing using the two RRO correction amounts respectively learned atthe linear learning positions px11 and px21 in a case where thelinearity error is small. In the distribution D1 of the positioningerrors, each round point indicates a measured value of each positioningerror measured by positioning the head 15 at each radial position in thetrack TRk. In FIG. 7, the distribution D2 of the positioning errorsillustrates change of the positioning error in a case of positioning thehead 15 at the radial positions of the track TRk based on the RROcorrection amounts corresponding to the radial positions of the trackTRk acquired by the linear RRO correction processing using the two RROcorrection amounts respectively learned at the linear learning positionspx11 and px21 in a case where the linearity error is large. In thedistribution D2 of the positioning errors, each cross point indicates ameasured value of each positioning error measured by positioning thehead 15 at each radial position in the track TRk. In FIG. 7, thedistribution D3 of the positioning errors illustrates change of thepositioning error in a case of positioning the head 15 at the radialpositions of the track TRk based on the RRO correction amountscorresponding to the radial positions of the track TRk acquired by thelinear RRO correction processing using the two RRO correction amountsrespectively learned at the linear learning positions px12 and px22 in acase where the linearity error is large. In the distribution D3 of thepositioning errors, each square point indicates a measured value of eachpositioning error measured by positioning the head 15 at each radialposition in the track TRk.

In the example illustrated in FIG. 7, in the case where the linearityerror is large, the positioning errors are smaller in the linearlearning positions px12 and px22 than in the linear learning positionspx11 and px21. In other words, in the case where the linearity error islarge, the positioning error becomes smaller as an interval between twolinear learning positions (hereinafter referred to as linear learningposition interval) is smaller. That is, in a case where the linearityerror is large, the cycle of the variation of RRO is short and thevariation of RRO is large. As illustrated in FIG. 7, the linearity errorcorresponds to RRO (RRO correction amount). For example, in a case ofexecuting the linear RRO correction processing, the RRO learning unit620 makes the linear learning position interval smaller as the linearityerror becomes larger. In other words, in the case of executing thelinear RRO correction processing, the RRO learning unit 620 makes thelinear learning position interval larger as the linearity error becomessmaller. Further, for example, in the case of executing the linear RROcorrection processing, the RRO learning unit 620 can maximize the linearlearning position interval when the linearity error is zero.

FIG. 8 is a diagram illustrating an example of a method of setting alinear learning position according to the present embodiment. In FIG. 8,the horizontal axis represents gamma (γ) corresponding to the correctionparameter corresponding to a particular track, and the vertical axisrepresents the linear learning position corresponding to the distance inthe radial direction from the track center of the particular track. Thelinear learning position is, for example, on the order of nanometers[nm]. On the horizontal axis, the gamma becomes larger in the directionof positive values as the axis goes along the positive arrow from theorigin 0, and the gamma becomes smaller in the direction of negativevalues as the axis goes along the negative arrow from the origin 0. Onthe vertical axis, the linear learning position becomes larger as theaxis goes along the arrow of large, and the linear learning positionbecomes smaller as the axis goes along the arrow of small. In FIG. 8,each of a plurality of round points indicates the linear learningposition measured with particular gamma. Hereinafter, the learningposition measured with the particular gamma is referred to as ameasurement point. FIG. 8 illustrates a correlation expression CLbetween the gamma and an optimum linear learning position, which isderived from a plurality of the measurement points. FIG. 8 illustrates aradial position LP1 corresponding to gamma γ1 in the correlationexpression CL and a radial position LP2 corresponding to gamma γ2 in thecorrelation expression CL. The gamma γ2 is larger than the gamma γ1. Theradial position LP2 is smaller than the radial position LP1. The gammais acquired at the time of executing the linearity correction, andchanges according to the result of the linearity correction. The gammaindicates whether the linearity error of the radial region is large orsmall. The linearity error of the radial region becomes larger as thegamma becomes larger, and the linearity error of the radial regionbecomes smaller as the gamma becomes smaller. The Lissajous waveform inthe radial region approaches a quadrilateral shape as the gamma becomeslarger, and the Lissajous waveform in the radial region approaches around shape as the gamma becomes smaller. A ratio of maximum amplitudeto minimum amplitude of the Lissajous waveform in the radial regionbecomes larger than 1 as the gamma becomes larger, and the maximumamplitude with respect to the minimum amplitude of the Lissajouswaveform in the radial region approaches 1 as the gamma becomes smaller.

The RRO learning unit 620 sets the linear learning position based on thegamma acquired in the linearity correction of the radial region and thecorrelation expression CL. The RRO learning unit 620 may derive, inadvance, a plurality of the correlation expressions CL adjusted for eachmagnetic disk and each head from a plurality of the measurement pointsmeasured in each magnetic disk device or each head, and record theplurality of correlation expressions CL in a particular recordingregion, for example, the disk 10 and the nonvolatile memory 80. In acase of setting the linear learning position, the RRO learning unit 620selects an appropriate correlation expression for a particular magneticdisk device or a particular head. In the example illustrated in FIG. 8,in a case of acquiring the gamma γ1, the RRO learning unit 620 sets anabsolute value of the linear learning position in the outward directionfrom the track center of a particular track to the radial position LP1,and sets an absolute value of the linear learning position in the inwarddirection from the track center to the radial position LP1, based on thegamma γ1 and the correlation expression CL. In a case of acquiring thegamma γ2, the RRO learning unit 620 sets an absolute value of the linearlearning position in the outward direction from the track center of aparticular track to the radial position LP2, and sets an absolute valueof the linear learning position in the inward direction from the trackcenter to the radial position LP2, based on the gamma γ2 and thecorrelation expression CL. Note that the RRO learning unit 620 may setthe linear learning position based on the Lissajous waveform. Forexample, the RRO learning unit 620 makes the linear learning positionfrom the track center of a particular track larger as the Lissajouswaveform approaches a round shape, and makes the linear learningposition from the track center of the particular track smaller as theLissajous waveform approaches a quadrilateral shape.

The RRO recorder 630 positions the head 15 at a particular radialposition and writes the RRO correction data acquired by the RRO learningin the particular servo region SV. The RRO recorder 630 writes at leastone RRO correction data in each servo region SV. The RRO recorder 630can adjust a readable radial width (hereinafter referred to asreproduction width) of the RRO correction data. The RRO recorder 630 canincrease or decrease the reproduction width according to an arrangementinterval of the RRO correction data and a write condition (for example,write current or write flying height), for example. In addition, thereproduction width can also be increased or decreased according todesign conditions such as the width of the write head 15W and the widthof the read head 15R. The RRO recorder 630 writes the RRO correctiondata such that the center position of the reproduction width of the RROcorrection data (hereinafter simply referred to as RRO correction data)is arranged within a particular range set in the radial direction fromthe track center, in which write of data is allowed in each track(hereinafter, the particular range is referred to as allowed range).

The position corrector 640 reads the RRO correction data (RRO bit)corresponding to a particular region in the circumferential direction ofa particular track (the particular region is referred to ascircumferential region), and corrects the head position to approach aparticular radial position in the circumferential region, for example,the track center based on the RRO correction amount acquired from theread RRO correction data, the learning position obtained by learning theRRO correction data, and the off-track amount from the track center ofthe circumferential region corresponding to the RRO correction data tothe head position in the circumferential region corresponding to the RROcorrection data. The position corrector 640 executes the linear RROcorrection processing of calculating change of the RRO correction amountof the radial region based on at least two RRO correction amountsrespectively learned at least two linear learning positions, andcorrecting the head position in the radial region based on thecalculated change of the RRO correction amount of the radial region.

FIG. 9 is a flowchart illustrating an example of a method of determiningthe learning position in the linear RRO correction processing accordingto the present embodiment.

The MPU 60 adjusts the correction parameters in the linearity correctionexecuted when demodulating the signal read in the servo region SV(B901). The MPU 60 records correction parameters in a particularrecording region, for example, the disk 10 and the nonvolatile memory 80(B902). The MPU 60 sets the linear learning position based on thecorrection parameters (B903), writes the RRO correction data learned atthe set linear learning position, and terminates the processing.

FIG. 10 is a flowchart illustrating an example of the method of settingthe linear learning position according to the present embodiment.

In the processing of B903 in FIG. 9, the MPU 60 acquires the gamma ofthe correction parameters (B1001). The MPU 60 calculates the linearlearning position based on the gamma and the correlation expression CL(B1002). The MPU 60 sets the calculated linear learning position (B1003)and proceeds to the processing of B903.

According to the present embodiment, the magnetic disk device 1 adjuststhe correction parameters in the linearity correction executed whendemodulating the signal read in the servo region SV, and sets the linearlearning position based on the gamma in the correction parameters andthe correlation expression CL. Therefore, the magnetic disk device 1 canimprove the accuracy of the linear RRO correction processing. Therefore,the magnetic disk device 1 can improve the servo positioning accuracy.

Next, magnetic disk devices according to modifications and anotherembodiment will be described. In the modifications and anotherembodiment, the same reference numerals are given to the same parts asthose in the above embodiment, and a detailed description thereof willbe omitted.

(First Modification)

A magnetic disk device 1 of a first modification is different from theabove-described embodiment in a method of setting a linear learningposition.

After executing linearity correction in a radial region, an RRO learningunit 620 measures variation in a linearity error in the radial regionwhere the linearity correction has been executed, for example, in twotracks adjacent in a radial direction, and sets an RRO learning positionbased on the measured variation in the linearity error.

FIG. 11 is a diagram illustrating an example of variation in thelinearity error. In FIG. 11, the horizontal axis represents the radialdirection and the vertical axis represents the linearity error of theradial region. On the horizontal axis, a radial position is located onan outer periphery side of a disk 10 as the axis goes along the arrow ofan outward direction, and the radial position is located on an innerperiphery side of the disk 10 as the axis goes along the arrow of aninward direction. On the vertical axis, the linearity error becomeslarger in the direction of positive values as the axis goes along thepositive arrow from the origin 0, and the linearity error becomessmaller in the direction of negative values as the axis goes along thenegative arrow from the origin 0. FIG. 11 illustrates, for example,variation in the linearity error in the radial region of two servotracks as illustrated in FIG. 3. FIG. 11 illustrates variation REL1 ofthe linearity error in the radial region and variation REL2 of thelinearity error smaller than the variation REL1 of the linearity errorin the radial region. FIG. 11 illustrates threshold values RE1 and RE2for setting the linear learning position. For example, absolute valuesof the threshold values RE1 and RE2 are the same. Note that the absolutevalues of the threshold values RE1 and RE2 may not be the same. Forexample, the example illustrated in FIG. 11 may corresponds to theexample illustrated in FIG. 7.

In the example illustrated in FIG. 11, after executing the linearitycorrection in the radial region, the RRO learning unit 620 measures thevariation REL1 in the linearity error in the radial region where thelinearity correction has been executed, for example, in two tracksadjacent in the radial direction. The RRO learning unit 620 determinesthat the variation REL1 in the linearity error is larger than thethreshold values RE1 and RE2, and sets a linear learning position A, forexample, px12 and px22 illustrated in FIG. 7.

After executing the linearity correction, the RRO learning unit 620measures the variation REL2 in the linearity error in the radial regionwhere the linearity correction has been executed. The RRO learning unit620 determines that the variation REL2 in the linearity error is smallerthan the threshold values RE1 and RE2, and sets a linear learningposition B larger than the linear learning position A, for example, px11and px21 illustrated in FIG. 7.

FIG. 12 is a flowchart illustrating an example of a method of settingthe linear learning position according to the first modification.

In the processing in B903 in FIG. 9, an MPU 60 measures the linearityerror in the radial region where the linearity correction has beenexecuted (B1201), and determines whether (the absolute value of) thelinearity error is larger than (an absolute value of) a threshold value(B1202). When the MPU 60 determines that the linearity error is largerthan the threshold value (YES in B1202), the MPU 60 sets the linearlearning position A (B1203) and proceeds to the processing in B903. Whenthe MPU 60 determines that the linearity error is smaller than thethreshold value (NO in B1202), the MPU 60 sets the linear learningposition B larger than the linear learning position A (B1204) andproceeds to the processing in B903.

According to the first modification, after executing the linearitycorrection in the radial region, the magnetic disk device 1 measures thelinearity error in the radial region where the linearity correction hasbeen executed, and sets the RRO learning position based on the measuredlinearity error. Therefore, the magnetic disk device 1 can improve servopositioning accuracy.

(Second Modification)

A magnetic disk device 1 of a second modification is different from theabove-described embodiment and first modification in a method of settinga linear learning position.

After executing linearity correction in a radial region, an RRO learningunit 620 detects a variation periodic component in the radial regionwhere the linearity correction has been executed, for example, in theradial region of a linearity error in two tracks adjacent in a radialdirection, detects at least one variation periodic component havinglarger variation periodic amplitude than a threshold value (hereinafterreferred to as an amplitude threshold value) of variation periodicamplitude of the variation periodic component, detects a variationperiodic component having a shortest variation period (hereinafterreferred to as a shortest periodic component) in the at least onevariation periodic component having the variation periodic amplitudelarger than the amplitude threshold value, and sets linear learningpositions, for example, two linear learning positions, at an intervalequal to or smaller than half of the variation period of the shortestperiodic component. The variation period corresponds to, for example, aposition in the radial direction. For example, the RRO learning unit 620acquires the periodic component of the linearity error in two adjacenttracks as a fast Fourier transform (FFT) spectrum.

FIG. 13 is a diagram illustrating an example of the variation periodiccomponent of the linearity error. In FIG. 13, the horizontal axisrepresents a reciprocal of the variation period of the linearity errorof the radial region (hereinafter simply referred to as the variationperiod), and the vertical axis represents the variation periodicamplitude. The reciprocal of the variation period is a value obtained bydividing the servo track pitch by the variation period. Here, one servotrack pitch corresponds to a distance in the radial direction betweentwo adjacent tracks in the radial direction. The horizontal axisillustrates 1/the variation period=0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10.On the vertical axis, the variation periodic amplitude becomes larger asthe axis goes along the arrow of large, and the variation periodicamplitude becomes smaller as the axis goes along the arrow of small.FIG. 13 illustrates variation periodic amplitude A1 and A2 and amplitudethreshold values Ath1 and Ath2. The variation periodic amplitude A1 islarger than the amplitude threshold value Ath1. The amplitude thresholdvalue Ath2 is larger than the variation periodic amplitude A1 and theamplitude threshold value Ath1. The variation periodic amplitude A2 islarger than the variation periodic amplitude A1 and the amplitudethreshold values Ath1 and Ath2. FIG. 13 illustrates a variation periodiccomponent T1 of the variation periodic amplitude A1 at the reciprocal 4(=0.25 servo track pitch) of the variation period, and a variationperiodic component T2 of the variation periodic amplitude A2 at thereciprocal 2 (=0.5 servo track pitch) of the variation period.

In the example illustrated in FIG. 13, the RRO learning unit 620 detectsthe variation periodic component T1 and the variation periodic componentT2, which are the variation periodic amplitude larger than the amplitudethreshold value Ath1. The RRO learning unit 620 detects the variationperiodic component T1 as the shortest periodic component from thevariation periodic component T1 and the variation periodic component T2.The RRO learning unit 620 sets the linear learning position at aninterval equal to or smaller than 0.125 servo track pitch that is equalto or less than half of the variation period 0.25 servo track pitch ofthe variation periodic component T1.

In the example illustrated in FIG. 13, the RRO learning unit 620 detectsthe variation periodic component T2 that is the variation periodicamplitude A2 larger than the amplitude threshold value Ath2. The RROlearning unit 620 detects the variation periodic component T2 as theshortest periodic component. The RRO learning unit 620 sets the linearlearning position at an interval equal to or smaller than 0.25 servotrack pitch that is equal to or less than half of the variation period0.5 servo track pitch of the variation periodic component T2.

FIG. 14 is a schematic diagram illustrating an example of a method ofsetting a linear learning position according to a second modification.In FIG. 14, the horizontal axis represents the radial direction. On thehorizontal axis, a radial position is located on an outer periphery sideof a disk 10 as the axis goes along the arrow of an outward direction,and the radial position is located on an inner periphery side of thedisk 10 as the axis goes along the arrow of an inward direction. FIG. 14illustrates two tracks TRm and TRm−1 adjacent in the radial direction.FIG. 14 illustrates a track center TRCm of the track TRm and a trackcenter TRCm−1 of the track TRm−1. FIG. 14 illustrates a shortestperiodic component FCL of a variation period MTP. In the shortestperiodic component FCL, a round point indicates the set linear learningposition. In FIG. 14, the two linear learning positions are set at aninterval equal to or less than ½ of the variation period MTP. In FIG.14, the two linear learning positions are separated in the radialdirection at a linear learning position interval Ld. In other words, thelinear learning position interval is the interval Ld. FIG. 14illustrates two linear learning positions px31 and px32. The linearlearning position px31 is separated by an off-track amount x in anoutward direction with respect to the track center TRCm. The linearlearning position px32 is separated in an inward direction with respectto the track center TRCm. The linear learning position px32 is separatedfrom the linear learning position px31 by the interval Ld in the inwarddirection.

In the example illustrated in FIG. 14, the RRO learning unit 620 setsthe two linear learning positions at an interval equal to or less than ½(MTP/2) of the variation period MTP of the shortest periodic componentFCL. For example, the RRO learning unit 620 sets the linear learningposition px31 separated in the outward direction by the off-track amountx with respect to the track center TRCm of the track TRm at a particularinterval equal to or less than the variation period MTP/2 around thetrack TRm. For example, the RRO learning unit 620 sets the radialposition separated by an off-track amount Ld−x in the inward directionwith respect to the track center TRCm calculated based on the off-trackamount x from the track center TRCm of the track TRm at the linearlearning position px31 and the linear learning position interval Ld, asthe linear learning position px32, at a particular interval equal orless than the variation period MTP/2 around the track TRm.

FIG. 15 is a flowchart illustrating an example of a method of setting alinear learning position based on linearity correction according to thesecond modification.

In the processing in B903 in FIG. 9, an MPU 60 measures the linearityerror in the radial region where the linearity correction has beenexecuted (B1501), detects the variation periodic component of themeasured linearity error (B1502), and detects at least one variationperiodic component having the variation periodic amplitude larger thanthe amplitude threshold value (B1503). The MPU 60 detects the shortestperiodic component in the at least one variation periodic componenthaving the variation periodic amplitude larger than the amplitudethreshold value (B1504), sets the linear learning positions, forexample, the two linear learning positions, at the interval equal to orless than half of the variation period of the shortest periodiccomponent (B1505), and proceeds to the processing in B903.

According to the second modification, after executing the linearitycorrection in the radial region, the magnetic disk device 1 measures thelinearity error in the radial region where the linearity correction hasbeen executed, detects the variation periodic component of the measuredlinearity error, and detects at least one variation periodic componenthaving the variation periodic amplitude larger than the amplitudethreshold value. The magnetic disk device 1 detects the shortestperiodic component in the at least one variation periodic componenthaving the variation periodic amplitude larger than the amplitudethreshold value, and sets the linear learning positions at the intervalequal to or less than half of the variation period of the shortestperiodic component. Therefore, the magnetic disk device 1 can improveservo positioning accuracy.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. A magnetic disk device comprising: a diskincluding a recording region including a servo sector; a head configuredto write data to the disk and read data from the disk; and a controllerconfigured to acquire a plurality of correction data for a repeatablerunout of the recording region, the correction data respectivelycorresponding to a plurality of measurement positions set based on afirst linearity error acquired by reading the servo sector, and tocorrect a position of the head based on the correction data, wherein thecontroller makes an interval of the measurement positions smaller as thefirst linearity error becomes larger.
 2. The magnetic disk deviceaccording to claim 1, wherein the controller sets the measurementpositions based on a parameter corresponding to the first linearityerror acquired when the servo sector is read.
 3. The magnetic diskdevice according to claim 2, wherein the controller sets the measurementpositions based on the parameter and a first expression indicating arelationship between the parameter and the measurement positions.
 4. Amagnetic disk device comprising: a disk including a recording regionincluding a servo sector; a head configured to write data to the diskand read data from the disk; and a controller configured to acquire aplurality of correction data for a repeatable runout of the recordingregion, the correction data respectively corresponding to a plurality ofmeasurement positions set based on a first linearity error acquired byreading the servo sector, and to correct a position of the head based onthe correction data, wherein the controller sets the measurementpositions based on a second linearity error acquired by reading theservo sector after correcting the first linearity error, sets themeasurement positions to a first measurement position in a case wherethe controller determines that the second linearity error is larger thana first threshold value, and sets the measurement positions to a secondmeasurement position larger than the first measurement position in acase where the controller determines that the second linearity error issmaller than the first threshold value.
 5. A magnetic disk devicecomprising: a disk including a recording region including a servosector; a head configured to write data to the disk and read data fromthe disk; and a controller configured to acquire a plurality ofcorrection data for a repeatable runout of the recording region, thecorrection data respectively corresponding to a plurality of measurementpositions set based on a first linearity error acquired by reading theservo sector, and to correct a position of the head based on thecorrection data, wherein the controller sets the measurement positionsbased on a second linearity error acquired by reading the servo sectorafter correcting the first linearity error and sets the measurementpositions based on a plurality of periodic components of the secondlinearity error.
 6. The magnetic disk device according to claim 5,wherein the controller sets the measurement positions based on a firstperiodic component having amplitude larger than a first threshold valueand having a shortest first period in the periodic components.
 7. Themagnetic disk device according to claim 6, wherein the controller setsthe measurement positions for every second period equal to or less thanhalf of the first period.